1. Field of the Invention
The present invention relates to polypropylene resin compositions, and particularly to polypropylene resin compositions suited for heat sealing applications.
2. Background of the Art
Polypropylene (“PP”) films are widely used as packaging materials, especially for food. Coating, laminating or coextruding a substrate film with a film made of a heat-sealable resin yields a heat sealable film. Materials traditionally used in heat sealing applications are isotactic random copolymers of propylene with at least one more C2-C20-α-olefin other than propylene, made by using Ziegler/Natta based (“ZN”) catalysts. In order to simplify the description, random copolymers of propylene and ethylene will be referred to as C3/C2 random copolymers, random copolymers of propylene and 1-butene will be referred to as C3/C4 random copolymers, and random copolymers of propylene, ethylene and 1-butene will be referred to as C3/C2/C4 random copolymers.
Random copolymers of propylene and one second higher α-olefin other than ethylene will be referred to as C3/Cx-Cy random copolymers where x indicates the minimum amount of carbon atoms said second higher α-olefin can be comprised of, and y indicates the maximum amount of carbon atoms said second higher α-olefin can be comprised of. For example, the term C3/C4-C8 random copolymers comprises C3/C4 random copolymers, C3/C5 random copolymers, C3/C6 random copolymers, C3/C7 random copolymers and C3/C8 random copolymers. Random copolymers of propylene, ethylene and one third higher α-olefin will be referred to as C3/C2/Cx-Cy random copolymers where x indicates the minimum amount of carbon atoms said third higher α-olefin can be comprised of, and y indicates the maximum amount of carbon atoms said third higher α-olefin can be comprised of. For example, the term 3/C2/C4-C6 random copolymers comprises C3/C2/C4 random copolymers, C3/C2/C5 random copolymers and C3/C2/C6 random copolymers.
In order to fulfill its function as a good heat-sealing agent, a resin should have a low seal initiation temperature (“SIT”). Also, as much material as possible should remain unmelted at higher temperatures in order to ensure dimensional integrity of the sealing layer on the substrate layer during processing. Furthermore, the presence of low molecular weight, amorphous material soluble in or extractable by organic solvents such as xylene and hexane, respectively, should not be too high in order to prevent contamination of food. Furthermore, the melt flow rate (MFR) of a sealing layer composition must not exceed a critical value. Otherwise, it cannot be processed to a film or a layer in a bi- or multi-layer film. Finally, the stiffness of a sealing layer composition, usually expressed in terms of the E-modulus, should be as high as possible in order to provide the film with mechanical strength.
Typical sealing-layer grades are C3/C2, C3/C4 and C3/C2/C4 random copolymers. A low SIT is related to large amounts of the sealing layer material melting at temperatures as low as possible, but above room temperature. In C3/C2, C3/C4 and C3/C2/C4 random copolymers, a low SIT is achieved by lowering the melting point through the introduction of comonomers into the PP backbone. In order to retain crystallinity at typical film processing temperatures, a portion as large as possible must remain unmelted at temperatures as high as possible. As known in the art, the mutually exclusive requirements of low SIT and large amounts of unmelted material at high temperatures can only be optimized by producing blends of two or more random copolymers. Typically, one component has a relatively high melting point and a relatively high degree of crystallinity whereas the other component has a lower melting point, a lower degree of crystallinity and a relatively high level of unwanted solubles/extractables. These solubles/extractables are of an amorphous consistency and of low molecular weight. Typically, the component with the higher melting point is a C3/C2, C3/C4 or a C3/C2/C4 random copolymer and the component with the lower melting point is either a C3/C2, C3/C4 or a C3/C2/C4 random copolymer with the exception that blends of two different C3/C2 random copolymers, that is where the first C3/C2 random copolymer exhibits a higher melting point and the second C3/C2 random copolymer exhibits a lower melting point, are typically not used because C3/C2 random copolymers contain higher levels of solubles/extractables compared to C3/C4 or C3/C2/C4 random copolymers and the use of two components with relatively high levels of solubles/extractables would lead to compositions with exorbitantly high levels of solubles/extractables.
EP 263 718-B1 relates to a low-crystalline propylene random copolymer compositions comprising blends of C3/C2/C4-C20 and C3/C4-C20 copolymers. The levels of xylene solubles and n-hexane extractables of these compositions need improvement.
EP 483 523-B1 relates to compositions based on crystalline propylene copolymers comprising blends of C3/C4-C8 and C3/C2/C4-C8 random copolymers or blends of C3/C4-C8 and C3/C2 random copolymers. The balance of the level of extractables, particularly hexane extractables, and the SIT of these compositions is not satisfactory. Furthermore, the compositions with low SIT need amounts of ≧35% of low crystallinity fraction. This limits large-scale production of such compositions to specialty processes as described in EP 483 523-B1 and in [P. Giusti, L. Lazzeri, N. Barbani, L. Lelli, S. DePetris, M. G. Cascone, Macromol. Symp. 78, 285-297 (1994)].
EP 560 326-B1 relates to semicrystalline polyolefin compositions comprising blends of C3/C4-C10 and C3/C4-C10 random copolymers. The balance between SIT and the level of solubles as well as the haze of these compositions need improvement. Furthermore, the compositions with low SIT need amounts of ≧35% of low crystallinity fraction. This limits large-scale production of such compositions to specialty processes as described above.
EP 674 991-B1 relates to crystalline propylene polymer compositions comprising C3/C2 and C3/C2/C4-C8 random copolymers. The level of hexane extractables and the SIT of these compositions need improvement. Furthermore, the compositions with low SIT need amounts of ≧35% of low crystallinity fraction. This limits large-scale production of such compositions to specialty processes as described above.
EP 780 432-B1 relates to compositions based on propylene polymers comprising blends of C3/C2/C4 and C3/C2/C4 random copolymers or blends of C3/C4 and C3/C2/C4 random copolymers. The balance between SIT and the level of hexane extractables of these compositions need improvement.
EP 881 239-B1 relates to C3/C2/C4 random copolymers. These grades need improvement in SIT.
WO 98/58971 relates to film-making C3/C2/C4-C8 random copolymers. These random copolymers need an improvement in SIT.
WO 00/11076 relates to crystalline propylene copolymer compositions comprising blends of C3/C2 or C3/C4-C8 or C3/C2/C4-C8 random copolymers with C3/C4-C8 or C3/C2/C4-C8 random copolymers. These compositions need improvement in the level of extractables, especially in hexane extractables. Furthermore, the resins obtained from the polymerization process of WO00/11076 need to be peroxidically visbroken in order to lower the amount of extractables to the desired level. This makes the process for their production more expensive than processes that do not require a peroxidic visbreaking step.
WO 02/44251 relates to the use of polymer compositions comprised of C3/C2/C4-C8 random copolymers for the production of films. Since these compositions are aiming at films with an improved balance of mechanical properties, the SIT is not competitive with the SIT of low sealing compositions. Moreover, the balance between tensile modulus and level of solubles needs improvement.
While the vast majority of sealing layer compositions is based on ZN catalysts, some compositions have been described that are based on metallocene catalysts. In order to simplify the description, the terms “produced by using metallocene catalysts” or “based on metallocene catalysts” will in the following be represented by the term “metallocene based”. For a good metallocene based sealing layer composition, the same principles apply as for a ZN based composition: the mutual exclusive requirements of low SIT and large amounts of unmelted material at high temperatures can only be optimized by producing blends of two or more random copolymers. It is known in the art that metallocene based copolymers contain lower levels of solubles than their ZN based analogs. Nonetheless, the introduction of amorphous, low molecular weight fractions into state-of-the-art metallocene based sealing layer compositions comprised of two or more components has been inevitable and is still too high with regard to food packaging applications.
In US application 2002/0176974-A1, heat-seal polymer films are disclosed comprising a layer of film formed from a metallocene based isotactic C3/C2 random copolymer. They need improvement in SIT.
In EP 982 328-B1, polypropylene resin compositions are disclosed comprising a polypropylene component and a C3/C2 random copolymer component. The main purpose of those compositions is their use as sealants in a broader sense, for example as heat-seal improving agents. However, they are unsuitable for acting as a stand-alone sealing-layer (as part of a bi- or multi-layer cast film or biaxially oriented film) because of their high MFR and high levels of extractables. Furthermore, such compositions contain from 50 to 99 wt % of the C3/C2 random copolymers. This limits large-scale production of such compositions to specialty processes as described above or in EP 982 328-B1.
An objective of the invention is to eliminate the disadvantages of the state-of-the-art and make polypropylene resin compositions available that are suited for heat-sealing applications and that have, at comparable SIT, lower levels of hexane extractables than state-of-the-art polypropylene compositions or that have, at comparable levels of hexane extractables, lower SITs than state-of-the-art polypropylene compositions or that have a combination of lower SIT and lower levels of extractables than state-of-the-art polypropylene compositions.
The objective of the invention is achieved by polypropylene compositions that exhibit xylene soluble fractions with very high molecular weight, and by polypropylene compositions that comprise ultra-low crystallinity fractions, and also by a process for preparing the polypropylene compositions.